1. Field of the Invention
The present invention relates to a noncontact information medium and a communication system that use the noncontact information medium. More specifically, the invention relates to a noncontact information medium capable of accurately performing communications even when a plurality of noncontact information media are overlapped, and a communication system that uses the noncontact information medium.
2. Description of the Related Art
An information medium is classified into a contact type or a noncontact type according to the communications method employed by the IC chip included in the medium to communicate with a reader-writer. Among these, the noncontact information medium does not cause any contact failure because it does not contact with the reader-writer and can be used movably and remotely from the reader-writer. In addition, the noncontact information medium is characterized by high resistance against dust, rain, and static electricity and by higher security. As a result, demand for the noncontact information medium is rising.
A typical noncontact information medium operates as follows. The noncontact information medium receives electromagnetic waves from a reader-writer and obtains operating power by electromagnetic induction. The noncontact information medium transmits data to and receives data from the reader-writer via radio waves. The noncontact information medium and the reader-writer include an antenna for transmitting and receiving the radio waves.
FIG. 12 is a schematic of a conventional noncontact information medium. A conventional noncontact information medium 100 includes a coil 102 acting as an antenna that receives power from outside and that transmits and receives data, a capacitor 104 that forms, together with the coil 102, a resonance circuit, and an IC chip 105 that controls an operation of the noncontact information medium 100. The noncontact information medium 100 has the IC chip 105 arranged generally in a central portion and the coil 102 provided around the IC chip 105.
To allow this noncontact information medium 100 to operate, the noncontact information medium 100 is brought closer to a reader-writer that transmits radio waves for transmission and reception. As a result, the coil 102 and the capacitor 104 of the noncontact information medium 100 resonate to generate an induced electromotive force, thereby carrying an induced current across the coil 102. This inducted current serves as a power for a power supply of the IC chip 105, thereby allowing the IC chip 105 to operate and the noncontact information medium 100 to transmit data to the reader-writer via the coil 102. Thus, electromagnetic coupling between the reader-writer and the coil 102 of the noncontact information medium 100 enables transmission and reception of data between the noncontact information medium 100 and the reader-writer (see Japanese Patent Application Laid-open No. 2001-34725).
The conventional noncontact information medium is used under the assumption that one noncontact information medium does not interfere with a different noncontact information medium. However, when a plurality of noncontact information media are close to the reader-writer or when a plurality of noncontact information media overlap, data transmission and reception cannot be accurately performed. As shown in FIG. 13, when two the noncontact information media 100 of same size are overlapped, in particular, then the coils 102 serving as their respective antennas are located at the same position, and two the noncontact information media 100 interfere with one another. This results in disturbance of a resonance frequency, an unstable communication state, and an inability of holding a communication.
Furthermore, when power is fed from the reader-writer to each of a plurality of noncontact information media within a communicable area, it is necessary for the reader-writer to have a substantial feeding ability so as to enable communicating with all the noncontact information media. When the number of the noncontact information media is large, however, the reader-writer is unable to efficiently feed power to all the noncontact information media.
The present invention has been achieved in view of the problems in the conventional technology. It is an object of the invention to provide a noncontact information medium that can perform communication even when a plurality of noncontact information media overlap, and a communication system that uses the noncontact information medium.